Glazing requirements have changed radically in recent years. In particular, modern commercial buildings and multi-dwelling buildings employ large glass panes and curtain walls.
The development of the curtain wall in the 1940's produced new requirements in glazing technology.
A curtain wall is essentially non-load bearing, carries its own weight, is usually hung from the super-structure and is subject to structural movement.
Glazing materials employed in such structures must be able to accommodate structural movements of mechanical origin developed by the wind, and thermal origin developed by expansion and contraction of the frame structure.
So-called "wet" glazing systems have been widely employed which comprise an elongated mass of tacky mastic material which is temporarily supported on an elongated strip of paper. The mastic material adheres to the window pane and to the window mounting structure and provides an effective, water-tight seal. On the other hand, the flowable nature of the mastic causes it to flow out onto the window pane in streaks, in response to structural movements which exert pressure on it. These streaks are unsightly and interfere with the clear view otherwise provided by the window pane.
In addition when streaking occurs on the outer window pane surfaces, rain washes the streaked mastic from the pane and onto the adjacent building structures, producing an overall deterioration in the appearance of the building.
More recently, so-called "dry" glazing has been developed which employs an extruded resilient is gasket, for example, a rubber gasket. These resilient gaskets do not exhibit streaking and produce a uniform edge around the pane which is more aesthetic in appearance than the edge produced by the mastic. The resilient gaskets are also easier to install.
Rubber gaskets provide a long-lasting weathertight seal but the sealing action is less effective than that of the mastic, which can flow into the surface irregularities of the frame structure. Consequently the "wet" glazing system has remained in wide spread use in spite of its inherent disadvantages.